/*
 * Copyright (C) 2012 Open Source Robotics Foundation
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
*/
#ifndef GZ_MATH_MATRIX4_HH_
#define GZ_MATH_MATRIX4_HH_

#include <algorithm>
#include <utility>
#include <gz/math/Helpers.hh>
#include <gz/math/Matrix3.hh>
#include <gz/math/Vector3.hh>
#include <gz/math/Pose3.hh>
#include <gz/math/config.hh>

namespace gz::math
{
  // Inline bracket to help doxygen filtering.
  inline namespace GZ_MATH_VERSION_NAMESPACE {
  //
  /// \class Matrix4 Matrix4.hh gz/math/Matrix4.hh
  /// \brief A 4x4 matrix class
  template<typename T>
  class Matrix4
  {
    /// \brief Identity matrix
    public: static const Matrix4<T> &Identity;

    /// \brief Zero matrix
    public: static const Matrix4<T> &Zero;

    /// \brief Constructor
    public: Matrix4()
    {
      memset(this->data, 0, sizeof(this->data[0][0])*16);
    }

    /// \brief Constructor
    /// \param[in] _v00 Row 0, Col 0 value
    /// \param[in] _v01 Row 0, Col 1 value
    /// \param[in] _v02 Row 0, Col 2 value
    /// \param[in] _v03 Row 0, Col 3 value
    /// \param[in] _v10 Row 1, Col 0 value
    /// \param[in] _v11 Row 1, Col 1 value
    /// \param[in] _v12 Row 1, Col 2 value
    /// \param[in] _v13 Row 1, Col 3 value
    /// \param[in] _v20 Row 2, Col 0 value
    /// \param[in] _v21 Row 2, Col 1 value
    /// \param[in] _v22 Row 2, Col 2 value
    /// \param[in] _v23 Row 2, Col 3 value
    /// \param[in] _v30 Row 3, Col 0 value
    /// \param[in] _v31 Row 3, Col 1 value
    /// \param[in] _v32 Row 3, Col 2 value
    /// \param[in] _v33 Row 3, Col 3 value
    public: constexpr Matrix4(T _v00, T _v01, T _v02, T _v03,
                              T _v10, T _v11, T _v12, T _v13,
                              T _v20, T _v21, T _v22, T _v23,
                              T _v30, T _v31, T _v32, T _v33)
    : data{{_v00, _v01, _v02, _v03},
           {_v10, _v11, _v12, _v13},
           {_v20, _v21, _v22, _v23},
           {_v30, _v31, _v32, _v33}}
    {
    }

    /// \brief Construct Matrix4 from a quaternion.
    /// \param[in] _q Quaternion.
    public: explicit Matrix4(const Quaternion<T> &_q)
    {
      Quaternion<T> qt = _q;
      qt.Normalize();
      this->Set(1 - 2*qt.Y()*qt.Y() - 2 *qt.Z()*qt.Z(),
                2 * qt.X()*qt.Y() - 2*qt.Z()*qt.W(),
                2 * qt.X() * qt.Z() + 2 * qt.Y() * qt.W(),
                0,

                2 * qt.X() * qt.Y() + 2 * qt.Z() * qt.W(),
                1 - 2*qt.X()*qt.X() - 2 * qt.Z()*qt.Z(),
                2 * qt.Y() * qt.Z() - 2 * qt.X() * qt.W(),
                0,

                2 * qt.X() * qt.Z() - 2 * qt.Y() * qt.W(),
                2 * qt.Y() * qt.Z() + 2 * qt.X() * qt.W(),
                1 - 2 * qt.X()*qt.X() - 2 * qt.Y()*qt.Y(),
                0,

                0, 0, 0, 1);
    }

    /// \brief Construct Matrix4 from a math::Pose3
    /// \param[in] _pose Pose.
    public: explicit Matrix4(const Pose3<T> &_pose) : Matrix4(_pose.Rot())
    {
      this->SetTranslation(_pose.Pos());
    }

    /// \brief Change the values
    /// \param[in] _v00 Row 0, Col 0 value
    /// \param[in] _v01 Row 0, Col 1 value
    /// \param[in] _v02 Row 0, Col 2 value
    /// \param[in] _v03 Row 0, Col 3 value
    /// \param[in] _v10 Row 1, Col 0 value
    /// \param[in] _v11 Row 1, Col 1 value
    /// \param[in] _v12 Row 1, Col 2 value
    /// \param[in] _v13 Row 1, Col 3 value
    /// \param[in] _v20 Row 2, Col 0 value
    /// \param[in] _v21 Row 2, Col 1 value
    /// \param[in] _v22 Row 2, Col 2 value
    /// \param[in] _v23 Row 2, Col 3 value
    /// \param[in] _v30 Row 3, Col 0 value
    /// \param[in] _v31 Row 3, Col 1 value
    /// \param[in] _v32 Row 3, Col 2 value
    /// \param[in] _v33 Row 3, Col 3 value
    public: void Set(
          T _v00, T _v01, T _v02, T _v03,
          T _v10, T _v11, T _v12, T _v13,
          T _v20, T _v21, T _v22, T _v23,
          T _v30, T _v31, T _v32, T _v33)
    {
      this->data[0][0] = _v00;
      this->data[0][1] = _v01;
      this->data[0][2] = _v02;
      this->data[0][3] = _v03;

      this->data[1][0] = _v10;
      this->data[1][1] = _v11;
      this->data[1][2] = _v12;
      this->data[1][3] = _v13;

      this->data[2][0] = _v20;
      this->data[2][1] = _v21;
      this->data[2][2] = _v22;
      this->data[2][3] = _v23;

      this->data[3][0] = _v30;
      this->data[3][1] = _v31;
      this->data[3][2] = _v32;
      this->data[3][3] = _v33;
    }

    /// \brief Set the upper-left 3x3 matrix from an axis and angle
    /// \param[in] _axis the axis
    /// \param[in] _angle ccw rotation around the axis in radians
    public: void Axis(const Vector3<T> &_axis, T _angle)
    {
      T c = cos(_angle);
      T s = sin(_angle);
      T C = 1-c;

      this->data[0][0] = _axis.X()*_axis.X()*C + c;
      this->data[0][1] = _axis.X()*_axis.Y()*C - _axis.Z()*s;
      this->data[0][2] = _axis.X()*_axis.Z()*C + _axis.Y()*s;

      this->data[1][0] = _axis.Y()*_axis.X()*C + _axis.Z()*s;
      this->data[1][1] = _axis.Y()*_axis.Y()*C + c;
      this->data[1][2] = _axis.Y()*_axis.Z()*C - _axis.X()*s;

      this->data[2][0] = _axis.Z()*_axis.X()*C - _axis.Y()*s;
      this->data[2][1] = _axis.Z()*_axis.Y()*C + _axis.X()*s;
      this->data[2][2] = _axis.Z()*_axis.Z()*C + c;
    }

    /// \brief Set the translational values [ (0, 3) (1, 3) (2, 3) ]
    /// \param[in] _t Values to set
    public: void SetTranslation(const Vector3<T> &_t)
    {
      this->data[0][3] = _t.X();
      this->data[1][3] = _t.Y();
      this->data[2][3] = _t.Z();
    }

    /// \brief Set the translational values [ (0, 3) (1, 3) (2, 3) ]
    /// \param[in] _x X translation value.
    /// \param[in] _y Y translation value.
    /// \param[in] _z Z translation value.
    public: void SetTranslation(T _x, T _y, T _z)
    {
      this->data[0][3] = _x;
      this->data[1][3] = _y;
      this->data[2][3] = _z;
    }

    /// \brief Get the translational values as a Vector3
    /// \return x,y,z translation values
    public: Vector3<T> Translation() const
    {
      return Vector3<T>(this->data[0][3], this->data[1][3], this->data[2][3]);
    }

    /// \brief Get the scale values as a Vector3<T>
    /// \return x,y,z scale values
    public: Vector3<T> Scale() const
    {
      return Vector3<T>(this->data[0][0], this->data[1][1], this->data[2][2]);
    }

    /// \brief Get the rotation as a quaternion
    /// \return the rotation
    public: Quaternion<T> Rotation() const
    {
      Quaternion<T> q;
      /// algorithm from Ogre::Quaternion<T> source, which in turn is based on
      /// Ken Shoemake's article "Quaternion Calculus and Fast Animation".
      T trace = this->data[0][0] + this->data[1][1] + this->data[2][2];
      T root;
      if (trace > 0)
      {
        root = sqrt(trace + 1.0);
        q.SetW(root / 2.0);
        root = 1.0 / (2.0 * root);
        q.SetX((this->data[2][1] - this->data[1][2]) * root);
        q.SetY((this->data[0][2] - this->data[2][0]) * root);
        q.SetZ((this->data[1][0] - this->data[0][1]) * root);
      }
      else
      {
        static unsigned int s_iNext[3] = {1, 2, 0};
        unsigned int i = 0;
        if (this->data[1][1] > this->data[0][0])
          i = 1;
        if (this->data[2][2] > this->data[i][i])
          i = 2;
        unsigned int j = s_iNext[i];
        unsigned int k = s_iNext[j];

        root = sqrt(this->data[i][i] - this->data[j][j] -
                    this->data[k][k] + 1.0);

        T a, b, c;
        a = root / 2.0;
        root = 1.0 / (2.0 * root);
        b = (this->data[j][i] + this->data[i][j]) * root;
        c = (this->data[k][i] + this->data[i][k]) * root;

        switch (i)
        {
          default:
          case 0: q.SetX(a); break;
          case 1: q.SetY(a); break;
          case 2: q.SetZ(a); break;
        };
        switch (j)
        {
          default:
          case 0: q.SetX(b); break;
          case 1: q.SetY(b); break;
          case 2: q.SetZ(b); break;
        };
        switch (k)
        {
          default:
          case 0: q.SetX(c); break;
          case 1: q.SetY(c); break;
          case 2: q.SetZ(c); break;
        };

        q.SetW((this->data[k][j] - this->data[j][k]) * root);
      }

      return q;
    }

    /// \brief Get the rotation as a Euler angles
    /// \param[in] _firstSolution True to get the first Euler solution,
    /// false to get the second.
    /// \return the rotation
    public: Vector3<T> EulerRotation(bool _firstSolution) const
    {
      Vector3<T> euler;
      Vector3<T> euler2;

      T m31 = this->data[2][0];
      T m11 = this->data[0][0];
      T m12 = this->data[0][1];
      T m13 = this->data[0][2];
      T m32 = this->data[2][1];
      T m33 = this->data[2][2];
      T m21 = this->data[1][0];

      if (std::abs(m31) >= 1.0)
      {
        euler.Z(0.0);
        euler2.Z(0.0);

        if (m31 < 0.0)
        {
          euler.Y(GZ_PI / 2.0);
          euler2.Y(GZ_PI / 2.0);
          euler.X(atan2(m12, m13));
          euler2.X(atan2(m12, m13));
        }
        else
        {
          euler.Y(-GZ_PI / 2.0);
          euler2.Y(-GZ_PI / 2.0);
          euler.X(atan2(-m12, -m13));
          euler2.X(atan2(-m12, -m13));
        }
      }
      else
      {
        euler.Y(-asin(m31));
        euler2.Y(GZ_PI - euler.Y());

        euler.X(atan2(m32 / cos(euler.Y()), m33 / cos(euler.Y())));
        euler2.X(atan2(m32 / cos(euler2.Y()), m33 / cos(euler2.Y())));

        euler.Z(atan2(m21 / cos(euler.Y()), m11 / cos(euler.Y())));
        euler2.Z(atan2(m21 / cos(euler2.Y()), m11 / cos(euler2.Y())));
      }

      if (_firstSolution)
        return euler;
      else
        return euler2;
    }

    /// \brief Get the transformation as math::Pose
    /// \return the pose
    public: Pose3<T> Pose() const
    {
      return Pose3<T>(this->Translation(), this->Rotation());
    }

    /// \brief Set the scale
    /// \param[in] _s scale
    public: void Scale(const Vector3<T> &_s)
    {
      this->data[0][0] = _s.X();
      this->data[1][1] = _s.Y();
      this->data[2][2] = _s.Z();
      this->data[3][3] = 1.0;
    }

    /// \brief Set the scale
    /// \param[in] _x X scale value.
    /// \param[in] _y Y scale value.
    /// \param[in] _z Z scale value.
    public: void Scale(T _x, T _y, T _z)
    {
      this->data[0][0] = _x;
      this->data[1][1] = _y;
      this->data[2][2] = _z;
      this->data[3][3] = 1.0;
    }

    /// \brief Return true if the matrix is affine
    /// \return true if the matrix is affine, false otherwise
    public: bool IsAffine() const
    {
      return equal(this->data[3][0], static_cast<T>(0)) &&
        equal(this->data[3][1], static_cast<T>(0)) &&
        equal(this->data[3][2], static_cast<T>(0)) &&
        equal(this->data[3][3], static_cast<T>(1));
    }

    /// \brief Perform an affine transformation
    /// \param[in] _v Vector3 value for the transformation
    /// \param[out] _result  The result of the transformation. _result is
    /// not changed if this matrix is not affine.
    /// \return True if this matrix is affine, false otherwise.
    public: bool TransformAffine(const Vector3<T> &_v,
                                 Vector3<T> &_result) const
    {
      if (!this->IsAffine())
        return false;

      _result.Set(this->data[0][0]*_v.X() + this->data[0][1]*_v.Y() +
                  this->data[0][2]*_v.Z() + this->data[0][3],
                  this->data[1][0]*_v.X() + this->data[1][1]*_v.Y() +
                  this->data[1][2]*_v.Z() + this->data[1][3],
                  this->data[2][0]*_v.X() + this->data[2][1]*_v.Y() +
                  this->data[2][2]*_v.Z() + this->data[2][3]);
      return true;
    }

    /// \brief Return the determinant of the matrix
    /// \return Determinant of this matrix.
    public: T Determinant() const
    {
      T v0, v1, v2, v3, v4, v5, t00, t10, t20, t30;

      v0 = this->data[2][0]*this->data[3][1]
         - this->data[2][1]*this->data[3][0];
      v1 = this->data[2][0]*this->data[3][2]
         - this->data[2][2]*this->data[3][0];
      v2 = this->data[2][0]*this->data[3][3]
         - this->data[2][3]*this->data[3][0];
      v3 = this->data[2][1]*this->data[3][2]
         - this->data[2][2]*this->data[3][1];
      v4 = this->data[2][1]*this->data[3][3]
         - this->data[2][3]*this->data[3][1];
      v5 = this->data[2][2]*this->data[3][3]
         - this->data[2][3]*this->data[3][2];

      t00 =  v5*this->data[1][1] - v4*this->data[1][2] + v3*this->data[1][3];
      t10 = -v5*this->data[1][0] + v2*this->data[1][2] - v1*this->data[1][3];
      t20 =  v4*this->data[1][0] - v2*this->data[1][1] + v0*this->data[1][3];
      t30 = -v3*this->data[1][0] + v1*this->data[1][1] - v0*this->data[1][2];

      return t00 * this->data[0][0]
           + t10 * this->data[0][1]
           + t20 * this->data[0][2]
           + t30 * this->data[0][3];
    }

    /// \brief Return the inverse matrix.
    /// This is a non-destructive operation.
    /// \return Inverse of this matrix.
    public: Matrix4<T> Inverse() const
    {
      T v0, v1, v2, v3, v4, v5, t00, t10, t20, t30;
      Matrix4<T> r;

      v0 = this->data[2][0]*this->data[3][1] -
        this->data[2][1]*this->data[3][0];
      v1 = this->data[2][0]*this->data[3][2] -
        this->data[2][2]*this->data[3][0];
      v2 = this->data[2][0]*this->data[3][3] -
        this->data[2][3]*this->data[3][0];
      v3 = this->data[2][1]*this->data[3][2] -
        this->data[2][2]*this->data[3][1];
      v4 = this->data[2][1]*this->data[3][3] -
        this->data[2][3]*this->data[3][1];
      v5 = this->data[2][2]*this->data[3][3] -
        this->data[2][3]*this->data[3][2];

      t00 = +(v5*this->data[1][1] -
          v4*this->data[1][2] + v3*this->data[1][3]);
      t10 = -(v5*this->data[1][0] -
          v2*this->data[1][2] + v1*this->data[1][3]);
      t20 = +(v4*this->data[1][0] -
          v2*this->data[1][1] + v0*this->data[1][3]);
      t30 = -(v3*this->data[1][0] -
          v1*this->data[1][1] + v0*this->data[1][2]);

      T invDet = 1 / (t00 * this->data[0][0] + t10 * this->data[0][1] +
          t20 * this->data[0][2] + t30 * this->data[0][3]);

      r(0, 0) = t00 * invDet;
      r(1, 0) = t10 * invDet;
      r(2, 0) = t20 * invDet;
      r(3, 0) = t30 * invDet;

      r(0, 1) = -(v5*this->data[0][1] -
          v4*this->data[0][2] + v3*this->data[0][3]) * invDet;
      r(1, 1) = +(v5*this->data[0][0] -
          v2*this->data[0][2] + v1*this->data[0][3]) * invDet;
      r(2, 1) = -(v4*this->data[0][0] -
          v2*this->data[0][1] + v0*this->data[0][3]) * invDet;
      r(3, 1) = +(v3*this->data[0][0] -
          v1*this->data[0][1] + v0*this->data[0][2]) * invDet;

      v0 = this->data[1][0]*this->data[3][1] -
        this->data[1][1]*this->data[3][0];
      v1 = this->data[1][0]*this->data[3][2] -
        this->data[1][2]*this->data[3][0];
      v2 = this->data[1][0]*this->data[3][3] -
        this->data[1][3]*this->data[3][0];
      v3 = this->data[1][1]*this->data[3][2] -
        this->data[1][2]*this->data[3][1];
      v4 = this->data[1][1]*this->data[3][3] -
        this->data[1][3]*this->data[3][1];
      v5 = this->data[1][2]*this->data[3][3] -
        this->data[1][3]*this->data[3][2];

      r(0, 2) = +(v5*this->data[0][1] -
          v4*this->data[0][2] + v3*this->data[0][3]) * invDet;
      r(1, 2) = -(v5*this->data[0][0] -
          v2*this->data[0][2] + v1*this->data[0][3]) * invDet;
      r(2, 2) = +(v4*this->data[0][0] -
          v2*this->data[0][1] + v0*this->data[0][3]) * invDet;
      r(3, 2) = -(v3*this->data[0][0] -
          v1*this->data[0][1] + v0*this->data[0][2]) * invDet;

      v0 = this->data[2][1]*this->data[1][0] -
        this->data[2][0]*this->data[1][1];
      v1 = this->data[2][2]*this->data[1][0] -
        this->data[2][0]*this->data[1][2];
      v2 = this->data[2][3]*this->data[1][0] -
        this->data[2][0]*this->data[1][3];
      v3 = this->data[2][2]*this->data[1][1] -
        this->data[2][1]*this->data[1][2];
      v4 = this->data[2][3]*this->data[1][1] -
        this->data[2][1]*this->data[1][3];
      v5 = this->data[2][3]*this->data[1][2] -
        this->data[2][2]*this->data[1][3];

      r(0, 3) = -(v5*this->data[0][1] -
          v4*this->data[0][2] + v3*this->data[0][3]) * invDet;
      r(1, 3) = +(v5*this->data[0][0] -
          v2*this->data[0][2] + v1*this->data[0][3]) * invDet;
      r(2, 3) = -(v4*this->data[0][0] -
          v2*this->data[0][1] + v0*this->data[0][3]) * invDet;
      r(3, 3) = +(v3*this->data[0][0] -
          v1*this->data[0][1] + v0*this->data[0][2]) * invDet;

      return r;
    }

    /// \brief Transpose this matrix.
    public: void Transpose()
    {
      std::swap(this->data[0][1], this->data[1][0]);
      std::swap(this->data[0][2], this->data[2][0]);
      std::swap(this->data[0][3], this->data[3][0]);
      std::swap(this->data[1][2], this->data[2][1]);
      std::swap(this->data[1][3], this->data[3][1]);
      std::swap(this->data[2][3], this->data[3][2]);
    }

    /// \brief Return the transpose of this matrix
    /// \return Transpose of this matrix.
    public: Matrix4<T> Transposed() const
    {
      return Matrix4<T>(
      this->data[0][0], this->data[1][0], this->data[2][0], this->data[3][0],
      this->data[0][1], this->data[1][1], this->data[2][1], this->data[3][1],
      this->data[0][2], this->data[1][2], this->data[2][2], this->data[3][2],
      this->data[0][3], this->data[1][3], this->data[2][3], this->data[3][3]);
    }

    /// \brief Equal operator for 3x3 matrix
    /// \param _mat Incoming matrix
    /// \return itself
    public: const Matrix4<T> &operator=(const Matrix3<T> &_mat)
    {
      this->data[0][0] = _mat(0, 0);
      this->data[0][1] = _mat(0, 1);
      this->data[0][2] = _mat(0, 2);

      this->data[1][0] = _mat(1, 0);
      this->data[1][1] = _mat(1, 1);
      this->data[1][2] = _mat(1, 2);

      this->data[2][0] = _mat(2, 0);
      this->data[2][1] = _mat(2, 1);
      this->data[2][2] = _mat(2, 2);

      return *this;
    }

    /// \brief Multiplication assignment operator. This matrix will
    /// become equal to this * _m2.
    /// \param[in] _m2 Incoming matrix.
    /// \return This matrix * _mat.
    public: Matrix4<T> operator*=(const Matrix4<T> &_m2)
    {
      (*this) = (*this) * _m2;
      return *this;
    }

    /// \brief Multiplication operator
    /// \param[in] _m2 Incoming matrix
    /// \return This matrix * _mat
    public: Matrix4<T> operator*(const Matrix4<T> &_m2) const
    {
      return Matrix4<T>(
        this->data[0][0] * _m2(0, 0) +
        this->data[0][1] * _m2(1, 0) +
        this->data[0][2] * _m2(2, 0) +
        this->data[0][3] * _m2(3, 0),

        this->data[0][0] * _m2(0, 1) +
        this->data[0][1] * _m2(1, 1) +
        this->data[0][2] * _m2(2, 1) +
        this->data[0][3] * _m2(3, 1),

        this->data[0][0] * _m2(0, 2) +
        this->data[0][1] * _m2(1, 2) +
        this->data[0][2] * _m2(2, 2) +
        this->data[0][3] * _m2(3, 2),

        this->data[0][0] * _m2(0, 3) +
        this->data[0][1] * _m2(1, 3) +
        this->data[0][2] * _m2(2, 3) +
        this->data[0][3] * _m2(3, 3),

        this->data[1][0] * _m2(0, 0) +
        this->data[1][1] * _m2(1, 0) +
        this->data[1][2] * _m2(2, 0) +
        this->data[1][3] * _m2(3, 0),

        this->data[1][0] * _m2(0, 1) +
        this->data[1][1] * _m2(1, 1) +
        this->data[1][2] * _m2(2, 1) +
        this->data[1][3] * _m2(3, 1),

        this->data[1][0] * _m2(0, 2) +
        this->data[1][1] * _m2(1, 2) +
        this->data[1][2] * _m2(2, 2) +
        this->data[1][3] * _m2(3, 2),

        this->data[1][0] * _m2(0, 3) +
        this->data[1][1] * _m2(1, 3) +
        this->data[1][2] * _m2(2, 3) +
        this->data[1][3] * _m2(3, 3),

        this->data[2][0] * _m2(0, 0) +
        this->data[2][1] * _m2(1, 0) +
        this->data[2][2] * _m2(2, 0) +
        this->data[2][3] * _m2(3, 0),

        this->data[2][0] * _m2(0, 1) +
        this->data[2][1] * _m2(1, 1) +
        this->data[2][2] * _m2(2, 1) +
        this->data[2][3] * _m2(3, 1),

        this->data[2][0] * _m2(0, 2) +
        this->data[2][1] * _m2(1, 2) +
        this->data[2][2] * _m2(2, 2) +
        this->data[2][3] * _m2(3, 2),

        this->data[2][0] * _m2(0, 3) +
        this->data[2][1] * _m2(1, 3) +
        this->data[2][2] * _m2(2, 3) +
        this->data[2][3] * _m2(3, 3),

        this->data[3][0] * _m2(0, 0) +
        this->data[3][1] * _m2(1, 0) +
        this->data[3][2] * _m2(2, 0) +
        this->data[3][3] * _m2(3, 0),

        this->data[3][0] * _m2(0, 1) +
        this->data[3][1] * _m2(1, 1) +
        this->data[3][2] * _m2(2, 1) +
        this->data[3][3] * _m2(3, 1),

        this->data[3][0] * _m2(0, 2) +
        this->data[3][1] * _m2(1, 2) +
        this->data[3][2] * _m2(2, 2) +
        this->data[3][3] * _m2(3, 2),

        this->data[3][0] * _m2(0, 3) +
        this->data[3][1] * _m2(1, 3) +
        this->data[3][2] * _m2(2, 3) +
        this->data[3][3] * _m2(3, 3));
    }

    /// \brief Multiplication operator
    /// \param _vec Vector3
    /// \return Resulting vector from multiplication
    public: Vector3<T> operator*(const Vector3<T> &_vec) const
    {
      return Vector3<T>(
          this->data[0][0]*_vec.X() + this->data[0][1]*_vec.Y() +
          this->data[0][2]*_vec.Z() + this->data[0][3],
          this->data[1][0]*_vec.X() + this->data[1][1]*_vec.Y() +
          this->data[1][2]*_vec.Z() + this->data[1][3],
          this->data[2][0]*_vec.X() + this->data[2][1]*_vec.Y() +
          this->data[2][2]*_vec.Z() + this->data[2][3]);
    }

    /// \brief Get the value at the specified row, column index
    /// \param[in] _col The column index. Index values are clamped to a
    /// range of [0, 3].
    /// \param[in] _row the row index. Index values are clamped to a
    /// range of [0, 3].
    /// \return The value at the specified index
    public: inline const T &operator()(const size_t _row,
                const size_t _col) const
    {
      return this->data[clamp(_row, GZ_ZERO_SIZE_T, GZ_THREE_SIZE_T)][
                        clamp(_col, GZ_ZERO_SIZE_T, GZ_THREE_SIZE_T)];
    }

    /// \brief Get a mutable version the value at the specified row,
    /// column index
    /// \param[in] _col The column index. Index values are clamped to a
    /// range of [0, 3].
    /// \param[in] _row the row index. Index values are clamped to a
    /// range of [0, 3].
    /// \return The value at the specified index
    public: inline T &operator()(const size_t _row, const size_t _col)
    {
      return this->data[clamp(_row, GZ_ZERO_SIZE_T, GZ_THREE_SIZE_T)]
                       [clamp(_col, GZ_ZERO_SIZE_T, GZ_THREE_SIZE_T)];
    }

    /// \brief Equality test with tolerance.
    /// \param[in] _m the matrix to compare to
    /// \param[in] _tol equality tolerance.
    /// \return true if the elements of the matrices are equal within
    /// the tolerence specified by _tol.
    public: bool Equal(const Matrix4 &_m, const T &_tol) const
    {
      return equal<T>(this->data[0][0], _m(0, 0), _tol)
          && equal<T>(this->data[0][1], _m(0, 1), _tol)
          && equal<T>(this->data[0][2], _m(0, 2), _tol)
          && equal<T>(this->data[0][3], _m(0, 3), _tol)
          && equal<T>(this->data[1][0], _m(1, 0), _tol)
          && equal<T>(this->data[1][1], _m(1, 1), _tol)
          && equal<T>(this->data[1][2], _m(1, 2), _tol)
          && equal<T>(this->data[1][3], _m(1, 3), _tol)
          && equal<T>(this->data[2][0], _m(2, 0), _tol)
          && equal<T>(this->data[2][1], _m(2, 1), _tol)
          && equal<T>(this->data[2][2], _m(2, 2), _tol)
          && equal<T>(this->data[2][3], _m(2, 3), _tol)
          && equal<T>(this->data[3][0], _m(3, 0), _tol)
          && equal<T>(this->data[3][1], _m(3, 1), _tol)
          && equal<T>(this->data[3][2], _m(3, 2), _tol)
          && equal<T>(this->data[3][3], _m(3, 3), _tol);
    }

    /// \brief Equality operator
    /// \param[in] _m Matrix3 to test
    /// \return true if the 2 matrices are equal (using the tolerance 1e-6),
    ///  false otherwise
    public: bool operator==(const Matrix4<T> &_m) const
    {
      return this->Equal(_m, static_cast<T>(1e-6));
    }

    /// \brief Inequality test operator
    /// \param[in] _m Matrix4<T> to test
    /// \return True if not equal (using the default tolerance of 1e-6)
    public: bool operator!=(const Matrix4<T> &_m) const
    {
      return !(*this == _m);
    }

    /// \brief Stream insertion operator
    /// \param _out output stream
    /// \param _m Matrix to output
    /// \return the stream
    public: friend std::ostream &operator<<(
                std::ostream &_out, const gz::math::Matrix4<T> &_m)
    {
      for (auto i : {0, 1, 2, 3})
      {
        for (auto j : {0, 1, 2, 3})
        {
          if (!(i == 0 && j == 0))
            _out << " ";

          appendToStream(_out, _m(i, j));
        }
      }

      return _out;
    }

    /// \brief Stream extraction operator
    /// \param[in,out] _in input stream
    /// \param[out] _m Matrix4<T> to read values into
    /// \return the stream
    public: friend std::istream &operator>>(
                std::istream &_in, gz::math::Matrix4<T> &_m)
    {
      // Skip white spaces
      _in.setf(std::ios_base::skipws);
      T d[16];
      _in >> d[0] >> d[1] >> d[2] >> d[3]
          >> d[4] >> d[5] >> d[6] >> d[7]
          >> d[8] >> d[9] >> d[10] >> d[11]
          >> d[12] >> d[13] >> d[14] >> d[15];

      if (!_in.fail())
      {
        _m.Set(d[0], d[1], d[2], d[3],
               d[4], d[5], d[6], d[7],
               d[8], d[9], d[10], d[11],
               d[12], d[13], d[14], d[15]);
      }
      return _in;
    }

    /// \brief Get transform which translates to _eye and rotates the X axis
    /// so it faces the _target. The rotation is such that Z axis is in the
    /// _up direction, if possible. The coordinate system is right-handed,
    /// \param[in] _eye Coordinate frame translation.
    /// \param[in] _target Point which the X axis should face. If _target is
    /// equal to _eye, the X axis won't be rotated.
    /// \param[in] _up Direction in which the Z axis should point. If _up is
    /// zero or parallel to X, it will be set to +Z.
    /// \return Transformation matrix.
    public: static Matrix4<T> LookAt(const Vector3<T> &_eye,
        const Vector3<T> &_target, const Vector3<T> &_up = Vector3<T>::UnitZ)
    {
      // Most important constraint: direction to point X axis at
      auto front = _target - _eye;

      // Case when _eye == _target
      if (front == Vector3<T>::Zero)
        front = Vector3<T>::UnitX;
      front.Normalize();

      // Desired direction to point Z axis at
      auto up = _up;

      // Case when _up == Zero
      if (up == Vector3<T>::Zero)
        up = Vector3<T>::UnitZ;
      else
        up.Normalize();

      // Case when _up is parallel to X
      if (up.Cross(Vector3<T>::UnitX) == Vector3<T>::Zero)
        up = Vector3<T>::UnitZ;

      // Find direction to point Y axis at
      auto left = up.Cross(front);

      // Case when front is parallel to up
      if (left == Vector3<T>::Zero)
        left = Vector3<T>::UnitY;
      else
        left.Normalize();

      // Fix up direction so it's perpendicular to XY
      up = (front.Cross(left)).Normalize();

      return Matrix4<T>(
          front.X(), left.X(), up.X(), _eye.X(),
          front.Y(), left.Y(), up.Y(), _eye.Y(),
          front.Z(), left.Z(), up.Z(), _eye.Z(),
                0,      0,         0,        1);
    }

    /// \brief Underlying data pointer
    /// \remarks This method is intended for python bindings (numpy).
    /// \remarks The bounds-checking array subscript operator is much preferred
    /// for element access from C++.
    /// \return A pointer to the underlying data array.
    public: T* Data()
    {
      return this->data[0];
    }

    /// \brief The 4x4 matrix
    private: T data[4][4];
  };

  namespace detail {

    template<typename T>
    constexpr Matrix4<T> gMatrix4Identity(
        1, 0, 0, 0,
        0, 1, 0, 0,
        0, 0, 1, 0,
        0, 0, 0, 1);

    template<typename T>
    constexpr Matrix4<T> gMatrix4Zero(
        0, 0, 0, 0,
        0, 0, 0, 0,
        0, 0, 0, 0,
        0, 0, 0, 0);

  }  // namespace detail

  template<typename T>
  const Matrix4<T> &Matrix4<T>::Identity = detail::gMatrix4Identity<T>;

  template<typename T>
  const Matrix4<T> &Matrix4<T>::Zero = detail::gMatrix4Zero<T>;

  typedef Matrix4<int> Matrix4i;
  typedef Matrix4<double> Matrix4d;
  typedef Matrix4<float> Matrix4f;
  }  // namespace GZ_MATH_VERSION_NAMESPACE
}  // namespace gz::math
#endif  // GZ_MATH_MATRIX4_HH_
